Prefilled syringes, each preliminarily filled with a medicinal liquid, have been known. Such a prefilled syringe includes an outer cylinder provided with a discharge port at the tip (distal end) thereof, a gasket inserted in the outer cylinder, and a plunger connected to the gasket, with a medicinal liquid contained in a space surrounded by the outer cylinder and the gasket.
Upon injecting a medicinal liquid into, for example, a living body by use of the prefilled syringe, a needle assembly having a hollow needle is mounted onto the discharge port of the outer cylinder, and the prefilled syringe is used in such a mounted state (see, for example, Patent Document 1). Further, in the mounted state, the inside of the outer cylinder and the inside of the hollow needle communicate with each other. The needle assembly described in Patent Document 1 includes a hollow needle, a tubular protector movable along the axial direction of the hollow needle between a first position, at which a hollow needle portion ranging to the needlepoint is covered, and a second position at which the needlepoint is exposed, a tubular support member externally supporting the protector in a movable manner, and a coil spring biasing the protector in a direction away from the second position and toward the first position. In the needle assembly described in Patent Document 1, during use of the needle assembly, when the protector is moved from the first position to the second position and then returned again to the first position, an engaging piece (resilient piece) provided on the support member protrudes toward the interior thereof, and comes into engagement with a base end surface (proximal surface) of the protector. This ensures that even if one attempts to move the protector further toward the second position, movement of the protector is restricted because the protector is in engagement with the engaging piece. Thus, the protector is inhibited from moving toward the side of the second position (see FIG. 4 of Patent Document 1).
However, the needle assembly described in Patent Document 1 has the following problem. For example, before use (in an unused state) thereof, if the needle assembly is dropped by mistake, the impact upon dropping may cause the protector to move from the first position to the second position and then return again to the first position. In this case as well, as mentioned above, movement of the protector is restricted by the engaging piece of the support member. Therefore, even though the needle assembly has not yet been used, the protector located at the first position cannot be moved to the second position, so that the needlepoint cannot protrude from the protector. In other words, the needle assembly cannot be put to use.
Patent Document 1: U.S. Pat. No. 4,894,055